A Full Dimension Multiple Input Multiple Output (FD-MIMO) communication system may be implemented by a Frequency Division Duplexing (FDD) scheme and a Time Division Duplexing (TDD) scheme. Since a system of the TDD scheme uses channel reciprocity between uplink and downlink, there is no great pressure on channel feedback. However, when an FD-MIMO technology is applied to a system of the FDD scheme used in many cellular networks, it is required to feedback channel state information to support beamforming at a transmitting end.
In general, the channel feedback is expressed by a finite number of feedback bits. When feedback information is incomplete, the beamforming at the transmitting end must be also inaccurate. In actual, when beamforming is not accurately performed in the multi-user MIMO system, inter-user interference is not completely removed and, as a result, a Signal to Interference and Noise Ratio (SINR) of a receiver is deteriorated. Accuracy of the channel state information directly influences a downlink multiplexing gain in the multi-user MIMO system. Particularly, in order to maintain a sum rate difference between complete channel state information and inaccurate channel state information within a predetermined range, the number of feedback bits for quantizing channels should be set in proportion to the number of Base Station (BS) antennas and a signal to noise ratio. The FD-MIMO system corresponds to a technology to acquire a high transmission rate through dozens to hundreds of antennas by the BS, and feedback load significantly increases in proportion to the number of antennas.
The performance of finite channel feedback is dependent on the accuracy of channel vector quantization. If beamforming is performed using an inaccurate quantized channel vector, inter-user interface is not completely removed in an interference limiting system and thus it is difficult to expect an increase in the sum rate. Particularly, in a case of the FD-MIMO communication system, as the number of transmission antennas increases, a dimension of a channel vector linearly increases. Accordingly, as the number of transmission antennas increases, an amount of feedback for quantizing the channel vector must increase. However, in order to not increase uplink overhead, the channel feedback should be expressed by as small a number of bits as possible and, accordingly, the accuracy of channel vector quantization is reduced and it is difficult to achieve a performance improvement through beamforming.